The semiconductor module of an electronic control unit (ECU) for a rotating electric machine may generate large amounts of heat during operation. As such, the ECU may have a radiator for dissipating heat produced by the semiconductor module. For example, in the ECU disclosed in a patent document 1 (i.e., Japanese Patent Laid-Open No. JP-A-2012-79741), the radiator is arranged on an opposite side of the semiconductor module relative to a substrate. That is, the substrate is interposed between the semiconductor module and the radiator. Therefore, the heat of the semiconductor module is transmitted to the radiator through the substrate.
In the above-described structure of the ECU in the patent document 1, the radiator has a board shape of a predetermined thickness to provide a large heat capacity. However, due to the substrate being interposed between the semiconductor module and the radiator, it is difficult to efficiently transfer heat from the semiconductor module to the radiator. Therefore, heat of the semiconductor module may be retained, thereby causing an abnormal operation of the semiconductor module.
Further, in the electronic control unit of the patent document 1, the semiconductor module has a cover member disposed on an opposite side of the radiator of the substrate. Heat of the semiconductor module is transmitted to the cover member through a heat conducting member. The cover member is intended to further facilitate a heat transfer during operation of the semiconductor module. However, the cover member has a small board thickness in comparison to the radiator. As a result, the heat capacity of the cover member is small. Therefore, heat transfer from the semiconductor module by the cover member in the patent document 1 may not be sufficient.